1. Field of the Invention
The present Invention relates to a contact material for vacuum circuit breakers, and in particular to a contact material in which weld resistance and voltage sustaining property are improved.
2. Description of the Prior Art
Contact materials for vacuum circuit breakers are basically required to have excellent material characteristics such as weld resistance, an ability to withstand preset voltage levels when contacts are in contact with each other, and an ability to completely prevent current from leaking across the contacts when the circuit is broken. It is further required that the temperature increase while making contact be small and that the contact resistance be stable at a low level. However, because some of these requirements run contrary to each other, it is difficult to meet all of the requirements by using a simple metal. Consequently, in most contact materials, two or more elements are combined in order to make up for the deficient properties of each individual element. In this way, the material characteristics are improved so that the contact material can be adapted for use in special conditions, such as heavy-currents, high-voltages and the like. Thus, these improved materials are superior to single-element materials. Up to now, however, a contact material with sufficient properties has not yet been found for handling recent trends which require the contacts to sustain heavier currents and higher voltages.
An example of a prior art contact material directed to heavy-current use is disclosed by Japanese Patent Publication No. S41-12131, in which a copper-bismuth alloy material includes a bismuth component as a weld inhibitor at a content of less than 5% by weight. However, in this Cu--Bi alloy material, the exceedingly low solubility of the Bi component in the Cu parent phase often gives rise to segregation of the Bi component In the alloy. As a result, the Cu--Bi alloy material has problems in that the contacting surfaces of the contacts made from this alloy become very rough quite easily, and it is difficult to shape and machine this alloy into contact parts.
On the other hand, another contact material for heavy-current use is disclosed in Japanese Patent publication No. S44-23751 in which a copper-tellurium alloy material is utilized. This alloy is free from the above-mentioned problems existing for the Cu--Bi alloy material, but, in comparison with the Cu--Bi alloy material, the Cu--Te alloy is more sensitive to the surrounding atmosphere, and the stability of the contact resistance is insufficient, etc.
Moreover, it has been discovered that the above-described Cu--Te and Cu--Bi alloy contact materials are equally unsatisfactory for adaptation to high-voltage, despite the fact that they have excellent weld resistant properties. In addition to that, their voltage withstanding properties are only sufficient for use at medium voltage levels.
As another contact material for a vacuum circuit breaker, a copper-chromium alloy material is known in the prior art. In this alloy material, the thermal characteristics of the Cr and Cu components are exhibited at a high temperature in a preferred manner for the contact material, and the properties of this alloy material are accordingly suitable for high-voltage and heavy-current use. Therefore, the Cu--Cr alloy material has been in widespread use because as it satisfies the requirements of both a high-voltage withstanding property and a large breaking capacity.
However, in regard to weld resistance, the above Cu--Cr alloy material is extremely inferior to the aforementioned Cu--Bi alloy material having a Bi component of less than 5%.
Here, referring to the welding phenomenon, it is considered that there are two occasions in which such phenomenon arises on the contacts. The first occasion is when the contact material resolidifies after belong melted at the contacting surfaces by Joule heat produced thereon. The second occasion is when the contact material is vaporized by arcing between the contacts at the moment when contact is being established or broken. On either occasion, the Cu and Cr components in the above-described Cu--Cr alloy material produce fine grains having a size of less than 1 .mu.m, which randomly mix with each other and form a layer having a thickness of a few .mu.m to a few hundred .mu.m.
Generally, the refining of material structures leads to increased material strength, and since the above Cu--Cr alloy material is not an exception, the strength of the fine-grain layer increases. As a result, if the strength of the refined Cu--Cr layer is greater than that of the matrix phase in the Cu--Cr alloy, and if the strength of the matrix phase exceeds the value of the mechanical power designed to be supplied to the contacts by an operating mechanism for breaking contact, then the welding phenomenon arises.
Therefore, in circuit breakers using the Cu--Cr alloy contact material, the operating mechanism must be designed so that a higher mechanical power is supplied for breaking contact than in the case of using a Cu--Bi alloy material. However, this is difficult in view of the needs of compactification and economy in the circuit breakers.
In response to the above problem, a copper-chromium-bismuth contact material has been proposed in Japanese Patent Publication No. 61-41091, which discloses a Cu--Cr alloy having an added Bi component for improving the weld resistance. This Improved material has better weld resistance, but becomes severely brittle due to the addition of the Bi component. Moreover, the voltage-withstanding property decreases and the restriking frequency increases.
Consequently, contact materials that are able to satisfy the various requirements mentioned above have not been provided by the prior art.